1. Field of the Invention
This disclosure relates to sewing machines in general and more particularly to sewing machines in which it is desirable to have a means for automatically advising the operator of the impending depletion of bobbin thread.
2. Description of the Prior Art
Typical low bobbin thread detection systems which operate from a photodetector sensor are known in the prior art and have frequently been of limited reliability due to the effect of stray light producing erroneous triggering of the low bobbin thread warning alarm. Most attempts to eliminate the effect of stray light from such sources as room lighting and sewing machine-mounted work-guiding lights have been limited to enclosing the loop taker cavity in which the light sensitive photodetector resides with an opaque bed slide and spraying the cavity with a flat black paint treatment to reduce internal reflections from reaching the photodetector. Another problem with some prior known low bobbin thread detector systems which utilize photodetectors is that the photodetector may not be conveniently aligned relative to the optical path during the manufacturing process. Still another problem is that some photodetector systems resort to complex electronic filter circuits to reduce the effects of stray light impinging on the photodetector. Still other prior known low bobbin thread detection systems have required the use of a specially designed bobbin which necessitated the production and stocking of more than one type of bobbin by sewing machine manufacturers.